Rudeness in church eating fish head in nave (9)
I believe the answer is:
insolence
'rudeness' is the definition.
(insolence is a kind of rudeness)
'in church eating fish head in nave' is the wordplay.
'church' becomes 'ce' (abbreviation for Church of England).
'eating' is an insertion indicator.
'fish' becomes 'sole' (I've seen this before**).
'head in' suggests taking the first letters.
The initial letter of 'nave' is 'n'.
'in'+'ce'='ince'
'sole'+'n'='solen'
'ince' going around 'solen' is 'INSOLENCE'.
(Other definitions for insolence that I've seen before include "Sauce" , "Lip; cheek" , "Rude behaviour" , "Rude disrespect" , "freshness" .)